1. Field of the Invention
The present invention relates to a paper feeding device for feeding paper sheets stacked on a vertically movable lifter.
2. Description of the Prior Art
In image recording apparatuses such as electrographic or xerographic copying machines or simple printers, paper sheets are generally fed by a method adopting paper cassettes. According to this method, a paper cassette containing paper sheets cut to predetermined lengths is inserted in a cassette insertion port of an image recording apparatus. The paper sheets stored in the cassette are individually fed into the apparatus by a paper feeding means and are supplied to an image recording section. When paper cassettes for separately storing paper sheets of different sizes, quality or the like are prepared, paper sheets to be supplied to the image recording apparatus may be easily switched by inserting or dismounting the cassettes. Alternatively, a plurality of cassettes storing paper sheets of different sizes can be mounted in a plurality of cassette insertion ports of an image recording apparatus, and the size of paper sheets to be fed to the apparatus may be switched by operation of a selection button.
However, a paper cassette can only store a maximum of, for example, 250 or 500 paper sheets. When a large number of copies such as, for example, 1,000 or 2,000 copies are to be reproduced continuously, paper replenishment must be performed several times during the copying operation, thus requiring frequent interruption of the copying operation. Due to such frequent paper replenishment, the recording apparatus cannot be used to its maximum capacity.
In view of this problem, some high-speed, high-quality recording apparatuses incorporate a deck (or tray) mechanism for storing a large number of paper sheets, for example, 1,000 paper sheets, and feeding these paper sheets. However, such a deck mechanism is usually built into the apparatus. Even if such a deck mechanism is detachable from the apparatus, signal lines and the like must be connected. Thus, the deck mechanism can only be used for an image recording apparatus for which it is designed, and cannot be used as a general purpose machine.
When individual paper sheets are individually supplied (fed or picked up hereinafter) from the paper feeding device to a main apparatus such as a printer, picking up of the paper sheets may be performed abnormally due to contact pressure between a or pickup roller and the paper sheets. For example, a paper feeding device is known wherein a large number of paper sheets are stacked on a paper lifter, and the position of the lifter is detected whereat the uppermost paper sheet on the lifter contacts the pickup roller with a constant pressure, thereby raising the lifter in accordance with the detection result obtained. In such a paper feeding device, picking up of a paper sheet may be performed even while the lifter is being raised. However, the operation of picking up only the uppermost paper sheet is greatly influenced by the surrounding physical conditions such as the presence/absence of vibration, the contact pressure between the pickup roller and the paper sheet, and the like. For this reason, reliable picking up of the paper sheets while the lifter is being raised is difficult, and erratic operation tends to occur.
A drive motor for a conventional vertically movable lifter is generally an AC motor. However, such an AC motor is bulky and renders the feeding device large in size. In addition to this, when the time difference between the stop time of the motor in the up control and that in the down control is large, and when fixing of the rotating shaft of the motor has as a reference the stop time of the motor in the down control, the rotating shaft is fixed after the motor is stopped and before the lifter begins to move downward. This results in a low fixed position precision and in degradation of or damage to the fixing device.
Furthermore, in a vertically movable lifter of this type, abnormal upward movement of the lifter is often detected. However, conventionally, when an abnormal upward movement of the lifter is detected, control operation of the lifter is simply terminated. For this reason, the lifter stops due to an abnormality such as flapping of the paper sheet, formation of a paper loop and the like, thereby interrupting processing for copying. If extra capability is to be built in the device so as to detect such flapping of the paper sheet, formation of a paper loop or the like without erratically interrupting control operation of the lifter, the feed efficiency of the device is impaired.
Detection of an abnormal upward movement of the lifter is generally performed using only switching contacts. The power source such as a motor for vertically moving the lifter is interrupted by the operation of the switch. Accordingly, if the switch operates erratically even when the motor is not driving the lifter to move it upward, the abnormality detecting means detects an abnormality, and again interrupts processing of the copying operation.
When a paper sheet fed from a lifter is to be stopped on a transfer section of the paper feed device, the stop position of the sheet is determined by a paper leading end sensor such as a switch which is located at a position corresponding to the leading end of a paper sheet properly stopped at the stop position. When a roller or the like of an image recording apparatus or the like is placed at the leading end side of the paper sheet being fed and the paper sheet must be properly brought into contact with the roller to be transferred thereby, correct transfer stop timing is hard to determine. The position of the trailing end of the paper sheet is also hard to determine. For these reasons, a second paper sheet can be fed only after a first paper sheet is completely fed into the recording apparatus or the like.
In a paper feeding device having a paper position sensor at a position corresponding to the trailing end of the fed paper sheet, a number of sensors must be incorporated to correspond to the number of different sizes of paper sheets which may be fed by the device, since the position of the trailing end of the paper sheet differs in accordance with the paper size.
Furthermore, since the stop position of a paper sheet is set with reference to an end of the paper sheet, the stop position cannot be determined with reference to a central portion of the paper sheet.
When a cassette storing a large number of paper sheets is mounted in place of a general cassette at a cassette mounting position, the copying machine performs display and control operations in accordance with the size of the paper sheets fed. For this reason, in order to perform normal copying the size of the paper sheets fed must be transmitted to the copying machine.